Manufacture of electroacoustic devices



Sept. 19; 1939. w. KALlN MANUFACTURE OF ELECTROACOUSTIC DEVICES 3Sheets-Sheet 1 Filed Oct. 19, 1938 lim m Wm mm .m WW w w W Sept. 19,1939. w. KALIN 2,173,481

MANUFACTURE OF ELECTROACOUSTIC DEVICES Filed Oct. 19, 1938 8Sheets-Sheet 2 IIIIIIIHIIIIHIIIHIHIl lllllIHIIIHIHIHIIIIIIIII llllllFIG...

//V l/E N TOR BY WKAL/N 4 TTORNEY' Sept. 19, 1939. w. mun 2,173,431

I MANUFACTURE OF ELECTROACOUSTIC nlwrcns Filed Oct. 19, 1938 3Shoots-Sheet 3 INVENTOR WKAL/N ATTORNEY Patented Sept. 19, 1939 PATENTOFFICE MAN UFA CTURE OF ELECTROACOUSTIC DEVICES Walter Kalin, Ramsey, N.1., assignor to Bell Telephone Laboratories, York, N. Y., a corporationIncorporated, New

of New York Application October 19, 1938, Serial No. 235,751

15 Claims.

This invention relates to the manufacture of electromagnetic. devicesand more particularly to the conditioning of electroacoustic devices,such as telephone receivers, of the permanent magnet type.

In high quality telephone receivers, such for example, as described inthe applications, Serial No. 93,792, filed August 1, 1936, and SerialNo. 161,936, filed September 1, 1937, 01' Louis A. Morrison and EdwardE. Mott, an important factor in the attainment of high efliciency anduniform response-characteristics is the utilization of the magneticmaterials in a manner enabling realization of their maximumcapabilities. It has been found highly desirable, in order that themaximum output of the receiver may be obtained, that the permanentmagnet flux be stabilized within a relatively narrow range. Because ofmanufacturing tolerances and variations in manufacturing conditions,attainment of the optimum flux values stit'uent elements of the receiveris impractical and economically unfeasible.

One general object of this invention is to enable conditioning ofelectromagnetic devices facilely and economically.

More specifically one object of this invention is to obtain facile,expeditious and economic stabilization of the permanent magnet flux inacoustic devices, such as telephone receivers, within the range ofvalues requisite for optimum performance characteristics.

Another object of this invention is to allow conditioning of permanentmagnet acoustic devices, such as telephone receivers, after the deviceshave been assembled. g

A further object of this invention is to obtain substantially uniformoperating characteristics for a multiplicity of telephone receivers ofsimilar construction.

In accordance with one feature of this invention, the permanent magnetor magnets of an electromagnetic device, such as a telephone receiver,is magnetized to a degree providing more flux than is requisite formaximum efliciency. The magnet is then subjected to demagnetizing forcesuntil the optimunr flux conditions are established.

- In accordance with another feature of this invention, the magnet isdemagnetized in successive small increments and control means areprovided for automatically causing cessation of the demagnetimtionaction when the permanent magnet flux is at substantially the optimumvalue.

In accordance with still another feature of the prior to assembly of theconelectrical output of invention, actuation of the control means iseffected directly in accordance with the output of the device, thepermanent magnet of which is being conditioned.

The invention and the foregoing and other I features thereof will beunderstood more clearly and fully from the following detaileddescription with reference to the accompanying drawings in which:

Fig. 1 is a general perspective view illustrating 10 the construction ofstabilizing apparatus for conditioning electromagnetic devices, such astelephone receivers in accordance with this invention;

Fig. 2 is a detail front view showing a portion 15 of the apparatus ofFig. 1 and the association of a telephone receiver therewith;

Fig. 3 is a side view of the apparatus illustrated in Fig. 2, portionsbeing shown in two positions assumed during operation;

I Fig. 4 is a fragmentary perspective view illustrating the manner inwhich electrical connection to the receiver is established when it is inthe stabilizing apparatus;

Fig. 5 is a diagrammatic view showing how a 26 telephone receiver may bemagnetized; and

Fig. 6 is a circuit diagram illustrating the electrical association ofthe various components of stabilizing apparatus illustrative of oneembodimentof this invention.

.Referring now to the drawings, Fig. 1 shows a general view of theapparatus as seen from the viewpoint of the one operating it. A box orcabinet l0 forms a convenient support for thoseparts of the apparatusdirectly under thev supervision of the operator. An acoustic coupler,one head ll of which is mounted in the top I2 of cabinet it, conveys theacoustic output of the receiver unit I3 to a sound pick-up means such asa microphone or another receiver unit, the

which is measured by means to be later described.

As maybe seen more clearly in Figs. 2 and 3 the receiver unit It isplaced diaphragm down on the coupler head II. A tube or duct, part of 46which is shown at It, connects the head I l to a similar head with whichthe sound pick-up means is associated.

The receiver for which the illustrated demagnetizer is designed is ofthe type having a perma- 50 nent magnet in the form of a straight bar.Such a receiver is shown diagrammatically inFig. 5 associated with amagnetizing means. The essential elements of this receiver are apermanent magnet I30 of bar type, pole pieces Ill attached tacts 40,shown in Fig. 4. The magnet M0 is usually made slightly oversize so thatwhen fully magnetized it will furnish more flux than that required formaximum receiver efficiency.

The demagnetizing means may take the form shown in detail in Figs. 2 and3 in which the demagnetizer is designated generally by it; A pair offrame members I1 of non-magnetic material, such as brass or bronze, aremounted on parallel supports i9 which are fitted to slide in the siderails iii mounted on the cabinet top iii. Supported between the framemembers it are pole members 29 and a magnetizing coil 2 i. The coil iiis wound on a suitable core of magnetic material (not shown) extendingbetween the pole members 20. Pole-pieces it are attached to pole members26 and are separated by a gap of suflicient width to accommodate thepermanent magnet portion of the receiver to be stabilized. Although thedemagnetizer illustrated is designed for handling receivers havingstraight bar type permanent magnets, the construction may be modified toaccommodate receivers having dii'lerently shaped magnets.

As shown in Fig. 3, the demagnetizer is movably mounted so that it maybe shifted between the operative position shown in full line and theinoperative position shown in broken line. This construction allows thereceiver to be conveniently and quickly inserted and removed and alsoallows for checking the receivers for maximum efiiciency afterstabilization without magnetic interference from the demagnetizer.

Electrical connections may be made to the contacts MI of receiver l3 bymeans of contactors 22, best seen in 4. Leads 49 and Eli are attached tothe contactors tor making the clrcuit connections shown in Fig. 6. Thecontactors 22 may be mounted on an operating handle or lever 23 whichmay be pivoted to a bracket 22 i supported on the cabinet I0. Lever 23may be biased to inoperative position by a leaf spring 25.

Other parts of the apparatus which may be mounted on the cabinet I0 arethe meter 26, starting switch control means 21 and potentiom eter knob28, the functions of which will appear more fully in the description tofollow. The cabinet! may be employed to house such other parts of theapparatus as may be conveniently contained therein. Connections may bemade to the rest of the apparatus including that on the panel 29 bymeans of a multi-conductor cable 30.

Mounted on panel 29 is a sequence switch denoted generally as 3|, whichmay be of conventional type. The one shown is driven by a motor 32 whichmay be controlled by a switch 33. In the embodiment shown the motor runscontinuously while the set is in use, connection to the sequence switchbeing made by way of a clutch 34 having an electromagnetic operatingmeans 36.

The means for magnetizing the receiver unit I3 may take any convenientform, one of which is shown diagrammatically in Fig. 5. The magnetizerI24 may comprise a coil I25 wound on a suitable core having pole-piecesI26 and I21. The receiver unit may be inserted between the polepiecesand switch I28 closed to supply magnetizing current from a source, suchas a battery I29. As has been previously noted, the receiver magnets aremade over-sized so that when they are lit connects to contact 13 whichcompletely magnetized the receiver is over-magnetized and below its bestefficiency.

The wiring diagram of Fig. 6 shows how the various elements described inthe foregoing may be interconnected with other means for performing thestabilizing operation.

The cam I94 of switch AI is shown in the position for stabilizing asindicated by the'notation STAB. When a re eiver unit I3 is positioned inthe demagnetizer I6 and handle 23 is depressed the receiver is connectedto a source of audio frequency 49. Source 48 may be a vacuum tubeoscillator or other suitable means and is connected to receiver it byconductors 69 and EU. Condensers 5i keep direct current out of theoscillator 48. A conductor i5i connects condoctor 50 to contact 52 ofswitch M. With the switch ii in stabilizing position as illustrated,contact 52 is connected to ground through contact 53. in the alternativeor test position of u switch M contact i'iii is connected to contact 5%which is connected by lead 55 to a center tap 03d of potentiometer 56.Conductor 49 is con nected by lead ill to choke coil 58, through switch59 when closed or ammeter 60 when switch 59 is opened, to a junctionpoint 6i. The circuit branches at St, one part going via lead 62 tocontact 63 and the other through variable resistor 69, adjustablecontact I38, and lead 65 to contact 66. With switch M in the stabilizingposition as shown, the circuit is from contact [55 to contact 61 vialead 68 to battery 69, thence via lead l9 to one end of potentiometer 56and ground. In the test position of switch 4i contact 6'2? isdisconnected from 66 and connected to contact ii, the circuit runningfrom 1! through lead 'iEto the other end of potentiometer 56. in thisposition of switch t! contact is connected to slider M of potentiometer563.

The sound pick-up means 15 has its electrical output connected to ameasuring circuit via a rectifier, such as the vacuum tube rectifier 16,through suitable amplifier and transformer means ii and 18,respectively. The rectified output is applied to a resistor i9 throughconductors 9d and 8G. The voltage across resistor 19 is employed tocharge condenser 82- by way oi the following circuit? One end ofresistor 19, resistor 83, condenser 82, lead 85, choke coil 86, leadill, contacts associated with cam D of sequence switch 39, lead 8%,contact 89 of switch H, to contact 90, when said switch is instabilizing position, lead 9i, back to the other end of resistor 19.

A discharge circuit for condenser 82 may be traced as follows: One sideof condenser 82, resistor 92, lead 93, contacts 94 and 95 of relay I00,lead 96, back to the other side of condenser 82.

The voltage drop across resistor 83 is applied to the grid circuit oftrigger tube 91 through the following circuit: One end of'resistor 93,lead 96, lead 91, variable'biasing resistor 98 to the cathode of tube91, grid of said tube and lead 99 back to the other end of resistor 93.

The anode voltage for tube 91 is supplied by a source, such as a batteryIOI, which has one pole connected to the cathode and the other throughlead I02, contacts associated with cam H of the sequence switch 3I, leadI03, coil of relay I00 to the anode. The heater is supplied with currentfrom a suitable source I04 which also supplies current to the cathode ofrectifier 16. u

A source of biasing potential such as battery I II is connected acrossresistor 00.

The clutch control magnet "receives current from grounded battery I05,through lead I00. The other side of the magnet 20 may be connected toground through various pathsduring the complete operating cycle of theset. With the sequence switch in the initial or No. 1 position, thispath is as follows: Magnet 30, lead I", contacts associated with cam 0,lead I00, switch I to ground, Switch I00 is normally biased to openposition being closed only to start the sequence switch. When thesequence switch has moved to position No. 2 the ground for magnet 05 isvia lead I01, upper contacts associated with cam B, lead IIO to ground.This connec- -tion is made for all even numbered positions of cam B. Afurther ground connection may be made to the lower contacts of cam Bwhich are closed from positions 2 to 20, inclusive, as indicated by thenotation 2/20, then via lead III,

contacts H2 and 0 of relay I00, switch II to ground. After the tube 0-!has fired and during testing, relay I00 is in the operated condition andcontacts H2, H0 are opened and an alternative ground is made from leadIII, via lead IIB, contacts II 0 and II I of switch I, to ground.

If it is desired to advance the sequence switch step by step duringstabilizing, the switch I I l is opened. Since contacts H0 and II! ofswitch I are also opened at this time there is no ground connection toconductor I I I. The ground connection is therefore through uppercontacts of cam B for even sequence switch positions and throughcontacts of cam C, lead I00, switch I00 to ground for odd positions ofthe sequence switch. Since switch I00 is normally biased to openposition the sequence switch stops at each odd position and must berestarted by closure of switch I00.

Resistor l0 and condenser 01 are connected across the demagnetizer I0;resistor H0 and condenser H9 across magnet means 05; condenser 00 acresresistor 00; condenser I20 across resistor choke 00 in lead 81 and chokeI2I in lead II to perform conventional smoothing, blocking and by-passfunctions.

For convenience of operation, switch I is mounted adjacent thedemagnetizer as shown in Fig. 3. Operating member l22 of switch 4| ,ismounted to cooperate with a projection or dog I on the demagnetizer I0whereby said switch is operated by movement of the demagnetizer to orfrom its operating position. Thus when the demagnetizer I0 is moved tothe forward or operating position switch II is positioned forstabilizins and when the demagnetizer is moved back, the switch is putin its test position.

The operation of the apparatus is generally as follows: Anovermagnetized receiver I0 is placed in the coupler and the demagnetizerI6 is moved to operative position. With switch II in stabilizingposition the sequence switch is started. On 4 the first step audiofrequency current from a source is applied to the speech current coilsof receiver I2. The sound output is converted into electric current bythe pick-up I0, amplified at the amplifier l1 and rectified at the fullwave rectifier I0, and the resulting voltage applied to charge condenser02. In the charging circuit is the resistor 00 which is connected acrossthe grid and cathode of the trigger tube 01. The grid is negativelybiased above the critical or breakdown voltage and resistor 00 is socon- 3 nected that its voltage drop during charging in-' creases thisbias. 1

The tube 01 may be one containing a gas, such as argon, in which theanode circuit becomes conducting only when the control electrode or gridis of a particular positive potential or less than a particular negativepotential and con tinues to pass current even though the controlelectrode voltage changes. Such a tube is ordinarily made non-conductingby the opening of the anode circuit by means outside the envelope.

When the sequence switch reaches its second position, the measuringcircuit is opened at the contacts associated with cam D and thedemagnetizer I 0 is energized by battery 31 through all of resistor 45.The resulting small current causes a partial demagnetization of thereceiver magnet. Since the receiver was initially overmagnetized andconsequently below peak emciency, its output will now be greater than atthe first step and on the third sequence switch step, which is ameasuring step, the charge on condenser 02 will be increased. Alternatedemagnetizing and measuring iscontinued, each successive demagnetizingstep being at a higher current, due to the cutting out of portions ofresistor 05 by action of cams E, F and G. A point in the demagnetizingsequence will be reached where the receiver output is no longerincreased but is slightly decreased. 0n the next measuring step theresulting rectified voltage will then be less than that on the condenser02 due to the charge of the previous step. The current which flows toequalize this voltage difference is in a direction through resistor 00to reduce the grid voltage of tube 01 below the critical value so thatthe tube may fire. The anode current of the tube 91 operates relay IIIto stop the sequence switch and discharge condenser 02. The sequenceswitch is stopped due to the opening of relay contacts 2 and III in thethen operative ground circuit for clutch operating magnet 35. Thiscircuit it will be recalled is from magnet 85 through lead Ill,"

upper and lower contacts on cam 13, lead III, contacts H2 and H3, switchill to ground. The alternative ground circuits for magnet 20 are open atthis step in the operation. The condenser 02 is discharged through thepreviously traced circuit 02, 92, 93, 90, 05, 06, 02 by closing ofcontacts 0i and 05. 'Demagnetizer I0 is then removed from receiver I3,which shifts the switch II to test position .by action of member I22 onswitch operator, I22 (see Fig. 3). Due to the closing of contacts 0 andII! of switch li a ground connection to magnet 00 is made through thepreviously traced circuit: 00, I01, B contacts, iii, H0, H0, H1, ground,the sequence switch is restarted andproceeds to its initial position.

Closure of test contacts 00 and I00 of switch I establishes a circuitfrom ground through meter 20, contacts I38 and 80, lead 0|, choke coil I2i through rectifier 10 back to ground at I01. This applies therectified out ut of sound pick-up 10 to meter 20. Closing 0 contacts 01and II in switch II connects center tapped potentiometer across battery09, the circuit being: 80, 00, 01, II, I2, 50, I0, 00. Contacts 08, I3and 52, 54 of switch 41 are also closed connecting the coils of receiverI3 between the center tap I00 and the slider 14 of potentiometer 00 overthe following. circuit: I00, 05, M, 5 2, III, 00, I3, 40, 01, 00, 00(normally closed) 0i, 02, 00; I3, 10.

With the foregoing test circuits established movement of potentiometerslider '14 to one side and then the other of its center position bymeans oi, knob, successively applies currents of opposite polarity tothe receiver coils. The unidirectional field of the receiver is thusincreased or reduced depending on current direction. If the receiver hasbeen conditioned to maximum efilciency either change in magnetizationwill reduce the output reading on meter 28. II more than a predeterminedallowable deviation from maximum efficiency is indicated on meter 26 thereceiver is rejected. removed and the cycle repeated with anotherreceiver.

When the sequence switch has returned to its initial or No. 1 position,the contacts associated with cam H are opened. This breaks the anodecircuit of tube 91 allowing the tube and relay I00 to return to'thenormal or unoperated condition.

Following the foregoing procedure, the receivers will be stabilized atsubstantially maximum' eillciency, being slightly oi-peak" on thedescending side of the efllciency curve, due to the necessity 01 passingbeyond maximum output to operate the trigger tube ill. on the efflciencycurve at which the trigger tube will operate, a small direct current maybe superposed onthe receiver windings. This current may be applied frombattery 69 through variable resistor 64, over the following circuit:ground; lead 10, battery 69, lead 58. contacts 66 andil (closed instabilizing position of switch -ot Switch H) .The latter may be M) lead65, slider I39, resistor 84, junction point it, meter (switch 55 beingopen) choke 59. leads ll and 4!. receiver ll, leads in and NH, contacts.52 and 53 (closed in stabilizing position to ground. After slider H9 isadjusted to apply the desired current which will be indicated on meter80. shunting switch 59 is. closed and remains closed for subsequentstabilizing operations. The adjustment may be such that the tube 91becomes conductive or fires right on the peak of the efficiency curve orslightly before maximum eilicienc'y is reached. desirable inthe-interests of stability in some receivers.

The sequence switch shown has twenty positions and seven operators orcams, designated bythe letters B to H, inclusive. The small numeralsadjacent each contact denote the position at which said contact isclosed. The designation 2/20 has been employed to indicate that theparticular contacts designated are closed for the positions 2 to 20inclusive. Cams E. F and Go! the switch II are connected in parallel asby conductor 36 and to a battery 31 by a common lead 38. nected byconductor 39 to one terminal of the demagnetizer IS, the other terminalof which is connected by conductor 42 to a contact 48 in switch it. Acooperating contact 43 of the switch is connected by lead 44 to one endof a resistor ll. A plurality of taps are taken from resistor 45 tocontacts of the sequence switch 3|. These taps are'so connected thatadvancement of the sequence switch cuts out portions of resistor I toapply successively higher current increments to demagnetizer l6.

The "foregoing detailed description is necessarily specificto theparticular details shown in the drawings, in order that the operation ofthe illustrative embodiment may be clear. It is believed obvious,however, that various changes in The receiver is then To controlthepoint- The other side of battery 31 is conform of apparatus andmanner of circuit connection may be made without departing from theinvention. For example, other means such as a relay chain could be usedin place of the sequence switch. In other words, various functionsancillary to the fundamental operation may be performed by suitableequivalents of the means particularly set forth. The scope of theinvention is limited not by the specific disclosure but by the appendedclaims only.

What is claimed is:

1. Apparatus for conditioning an electromagnetic device having apermanent magnet, said apparatus comprising means for indicating theoutput of said device, means for demagnetizing said magnet, and meanscontrolled by said indicating means for rendering said demagnetizingmeans inoperative when the output of said device is substantially atmaximum value.

2. Apparatus for conditioning an acoustic device having a permanentmagnet, comprising means for demagnetizing said magnet, output meansconnected to said device, a condenser in circuit with said output means,and means controlled by said condenser for controlling saiddemagnetizing means.

3. In apparatus for conditioning an acoustic device having a permanentmagnet, means in-' cluding an energization circuit for demagnetizingsaid magnet, an output circuit for said device, and means forcontrolling said demagnetizing means in accordance with the output ofsaid device comprising a space discharge device, an input circuit forsaid device, a condenser connected in said output circuit and to saidinput circuit, and an output circuit for said device including means forcontrolling said energization circuit.

Means for stabilizing the magnetization of permanent magnet means.included in an acoustic device, at a point corresponding to maximumeillciency of said device, comprising means for pp for measuring theoutput of the device, means for changing the magnetization of thepermanent magnet means to thereby adjust the efliciency of the device,and means responsive to the measuring means and connected to the meansfor changing the magnetization to deenergize the latter when maximumefllciency or the device is obtained.

5. A device for stabilizing permanent magnet acoustic devices comprisingmeans for overmagnetizing a device, means for partially demagnetlzingsaid device in successive small increments, means for applying audiofrequency power to said device, means responsive to the output of saiddevice, control means for rendering the demagnetizing means inoperative,and relay means connected between said control means and said outputresponsive means for stopping demagnetization at the point ofsubstantially maximum output.

6. Means for stabilizing the magnetization of the permanent magnet meansof a telephone receiver at a point corresponding to maximum receiverefllciency comprising means (or applying constant power to saidreceiver. means (or measuring the receiver output, means for changingthe magnetization of the permanent magnet means to thereby adjust thereceiver emciency, and means responsive to the measuring means andconnected to the means for changing the magnetization to deenergize thelatter when maximum receiver efllciency is obtained. 1

constant power to said device, means- 7. Means for stabilizing themagnetic circuit of a permanent magnet telephone receiver comprisingmeans for over-magnetizing said receiver, means for applying a currentof audible frequency to the voice coils of said receiver, means coupledto said receiver for measuring its sound output, means for partiallydemagnetlzing the overmagnetized receiver in successive smallincrements, relay means connected for stopp ng demagnetization whenmaximum sound output 0! the receiver is approximately obtained, andmeans operatably connecting said relay means for response to the soundoutput measuring means.

8. Means for stabilizing a permanent magnet type telephone receivercomprising means for overmagnetizing said receiver, means for applyingaudio frequency current to the voice coils of said receiver, meansresponsive to the output of said receiver for producing a direct currentvoltage varying directly with said output, magnetic means for partiallydemagnetizing said receiver in successive small increments, and meansresponsive to said direct current voltage, including a relay connectedto the magnetic means, tor stopping demagnetization substantially at thepoint of maximum direct current voltage.

9. In a stabilizing device for permanent magnet acoustic receivers,means for overmagnetizing a receiver, means including an electromagneticdemagnetizer for partially demagnetlzing said receiver in small steps,means for applying electric power to said receiver, means for measuringthe output of said receiver, and relay means responsive to saidmeasuring means and connected to control the demagnetizer, said relaymeans being adjusted to stop the demagnetization upon a slight reductionin output immediately following the peak output.

10. A magnetic stabilization means for a permanent magnet telephonereceiver comprising means for overmagnetizing said receiver, means forapplying an audio frequency current to the voice coils of the receiver,means responsive to the sound output of said receiver for producing adirect current voltage that is a direct function of said output, meansfor applying a demagnetizing force to the permanent magnet insuccessively greater increments to thereby incrementally increase thesound output of the receiver to a peak and then decrease it, relaymeans, and

means for connecting said relay means to respond to a value of thedirect current voltage occurring immediately after the peak output, saidrelay means being connected to thereupon stop the demagnetization means.

11. A device forstablizlng permanent magnet sound receivers comprisingmeans for overmagnetizing a receiver,-means for partially demagnetlzingthe receiver in successive small increments, means for applying audiofrequency current to said receiver, means responsive to the sound outputof said-receiver, control means for rendering the demagnetlzing meansimperative. and relay means connected between said control means andsaid sound responsive means for stopping demagnetization at the point ofsubstantial- 1y maximum sound output.

' 12. A device for stabilizing sound receivers as defined in claim 11 inwhich the sound responsive means includes means for producing a directcurrent voltage that is a direct function of the sound output, thecontrol means comprising an electromagnetic circuit controller and therelay means including a gas-filled electron tube of the trigger type.

13. Means for stabilizing a permanent magnet telephone receivercomprising means for overmagnetizing said receiver, means for applyingaudio frequency current to the voice coils of the receiver, means formeasuring the sound output of the receiver, means for demagnetlzing saidreceiver in successive small increments, control means for alternatelyconnecting the demagnetizing means and the measuring means to thereceiver, relay means responsive to the measuring means and connected tothe demagnetlzing means for .deenergizing the latter when substantiallymaximum output is measured, and means for adjusting the point at whichthe relay means responds comprising means for applying a biasingpotential to said voice coils.

14. A magnetic stabilizer for overmagnetized permanent magnet telephonereceivers comprising an oscillator connected to the voice coils of saidreceiver for applying a constant audio frequency current thereto, asound pick-up means, means for coupling the receiver output to the soundpick-up means, means for rectifying the output of the latter means, acondenser, a charging circuit for applying the resulting voltage tocharge said condenser, electromagnetic means applied to said receiverfor demagnetlzing the permanentmagnet, switching means including meansfor energizing the electromagnetic means and charging the condenser inalternate steps, means controlled by the switching means for increasingthe demagnetlzing effect of the electromagnetic means at each successivestep to thereby incrementally increase the receiver output to a maximumand then to decrease it, resistor means included in said chargingcircuit, a tube of the trigger type having its control electrodeconnected to said resistor and its anode circuit including a relayconnected to the electromagnetic means, said control electrode-resistorconnection being such that the relay is operated to deenergize theelectromagnetic means at the step immediately following that resultingin the maximum output of said receiver.

15. The method of stabilizing a permanent magnet acoustic device whichcomprises overmagnetizing said device, applying electric power to saiddevice and measuring its sound output, while partially demagnetlzing itin successive small increments, and stopping the demagnetizing whensubstantially maximum output is measured wanna nun.

